18,024 research outputs found
Arbitrary distribution and nonlinear modal interaction in coupled nanomechanical resonators
We propose a general one-dimensional {\em continuous} formulation to analyze
the vibrational modes of antenna-like nanomechanical resonators consisting of
two symmetric arrays of cantilevers affixed to a central nano-beam. The
cantilever arrays can have arbitrary density and length profile along the beam.
We obtain the secular equation that allows for the determination of their
frequency spectrum and illustrate the results on the particular examples of
structures with constant or alternating cantilever length profiles. We show
that our analytical results capture the vibration spectrum of such resonators
and elucidate key relationships that could prove advantageous for experimental
device performance. Furthermore, using a perturbative approach to treat the
nonlinear and dissipative dynamics of driven structures, we analyze the
anharmonic coupling between two specific widely spaced modes of the
coupled-element device, with direct application to experiments.Comment: 8 pages, 5 figures, additional info can be found at
http://nano.bu.ed
On a transition from solar-like coronae to rotation-dominated jovian-like magnetospheres in ultracool main-sequence stars
For main-sequence stars beyond spectral type M5 the characteristics of
magnetic activity common to warmer solar-like stars change into the brown-dwarf
domain: the surface magnetic field becomes more dipolar and the evolution of
the field patterns slows, the photospheric plasma is increasingly neutral and
decoupled from the magnetic field, chromospheric and coronal emissions weaken
markedly, and the efficiency of rotational braking rapidly decreases. Yet,
radio emission persists, and has been argued to be dominated by
electron-cyclotron maser emission instead of the gyrosynchrotron emission from
warmer stars. These properties may signal a transition in the stellar extended
atmosphere. Stars warmer than about M5 have a solar-like corona and
wind-sustained heliosphere in which the atmospheric activity is powered by
convective motions that move the magnetic field. Stars cooler than early-L, in
contrast, may have a jovian-like rotation-dominated magnetosphere powered by
the star's rotation in a scaled-up analog of the magnetospheres of Jupiter and
Saturn. A dimensional scaling relationship for rotation-dominated
magnetospheres by Fan et al. (1982) is consistent with this hypothesis
Imprint of spatial curvature on inflation power spectrum
If the universe had a large curvature before inflation there is a deviation
from the scale invariant perturbations of the inflaton at the beginning of
inflation. This may have some effect on the CMB anisotropy at large angular
scales. We calculate the density perturbations for both open and closed
universe cases using the Bunch-Davies vacuum condition on the initial state. We
use our power spectrum to calculate the temperature anisotropy spectrum and
compare the results with the WMAP three year data. We find that our power
spectrum gives a lower quadrupole anisotropy when , but matches
the temperature anisotropy calculated from the standard Ratra-Peebles power
spectrum at large . The determination of spatial curvature from temperature
anisotropy data is not much affected by the different power spectra which arise
from the choice of different boundary conditions for the inflaton perturbation.Comment: 17 pages, 4 figures, revtex4; section on comparison with WMAP3 data
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Effect of UV Radiation on the Spectral Fingerprints of Earth-like Planets Orbiting M dwarfs
We model the atmospheres and spectra of Earth-like planets orbiting the
entire grid of M dwarfs for active and inactive stellar models with =
2300K to = 3800K and for six observed MUSCLES M dwarfs with UV
radiation data. We set the Earth-like planets at the 1AU equivalent distance
and show spectra from the VIS to IR (0.4m - 20m) to compare
detectability of features in different wavelength ranges with JWST and other
future ground- and spaced-based missions to characterize exo-Earths. We focus
on the effect of UV activity levels on detectable atmospheric features that
indicate habitability on Earth, namely: HO, O, CH, NO and
CHCl.
To observe signatures of life - O/O in combination with reducing
species like CH, we find that early and active M dwarfs are the best
targets of the M star grid for future telescopes. The O spectral feature at
0.76m is increasingly difficult to detect in reflected light of later M
dwarfs due to low stellar flux in that wavelength region. NO, another
biosignature detectable in the IR, builds up to observable concentrations in
our planetary models around M dwarfs with low UV flux. CHCl could become
detectable, depending on the depth of the overlapping NO feature.
We present a spectral database of Earth-like planets around cool stars for
directly imaged planets as a framework for interpreting future lightcurves,
direct imaging, and secondary eclipse measurements of the atmospheres of
terrestrial planets in the HZ to design and assess future telescope
capabilities.Comment: in press, ApJ (submitted August 18, 2014), 16 pages, 12 figure
Low Temperature Fatigue Crack Growth Test and Life Estimation of 7475-T7351 Al-ally by ANN
In the present wok fatigue crack growth tests have been performed under interspersed mode-I overload on 7475-T7351 Al-alloy. The overloads with an overload ratio of 2 were given at 0C, ndash;30C, ndash;45C, ndash;60C, and ndash;75C at a loading rate of 7 KN/min after the crack had grown to a/w ratio of 0.4. The crack growth tests have been continued in mode-I at a frequency of 5 Hz and load ratio (R) of 0.1 till fracture. From the fatigue tests it has been observed that the crack growth rate decreases and consequently fatigue life increases as the overload temperature decreases. The experimental data generated have been subsequently used to formulate the ANN model to predict the fatigue crack growth rates and the fatigue life of 7475-T7351 Al-alloy. It has been observed that the proposed model predicts the fatigue life with reasonable accuracy having + 0.919% deviation from experimental results
Elliptic flow of thermal dileptons as a probe of QCD matter
We study the variation of elliptic flow of thermal dileptons with transverse
momentum and invariant mass of the pairs for Pb+Pb collisions at
= 2.76 TeV. The dilepton productions from quark gluon plasma
(QGP) and hot hadrons have been considered including the spectral change of
light vector mesons in the thermal bath. The space time evolution has been
carried out within the frame work of 2+1 dimensional ideal hydrodynamics with
lattice+hadron resonance gas equation of state. We find that a judicious
selection of invariant mass(M) and transverse momentum (p_T) windows can be
used to extract the collective properties of quark matter, hadronic matter and
also get a distinct signature of medium effects on vector mesons. Our results
indicate a reduction of elliptic flow (v_2) for M beyond phi mass, which if
observed experimentally would give the measure of v_2 of the partonic phase.Comment: To appear in Phys. Rev. C (Rapid Comm.
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